In satellite communication systems, the largest single source of positioning error to single frequency Global Positioning System (GPS) receivers is due to the unknown and large incurred time delay of the GPS signal as it propagates through the Earth's ionosphere. The ionosphere acts as a dispersive medium for radio signals. This delay is often referred to as Ionospheric group delay.
The Ionospheric group delay is defined by:
                                          τ            ⁡                          (              f              )                                ⁡                      [            m            ]                          =                              40.3            ·                          TEC              ⁡                              [                                  1                  ⁢                                      /                                    ⁢                                      m                    2                                                  ]                                                                        (                              f                ⁡                                  [                  Hz                  ]                                            )                        2                                              Equation        ⁢                                  [        1        ]            
where: f is the center frequency of the signal, and typical values are 1-100 TEC units (TECU), with 1 TECU=1016 electrons/m2.
There are two known methods to compensate for Ionospheric group delay. A first known method requires specialized infrastructure in the form of a network of ‘dual-frequency’ receivers located over the area of interest. In such a dual-frequency approach, a known method to compensate for distortion due to Ionospheric group delay is to transmit signals on the two frequency carriers, with one frequency carrier carrying an additional information signal that is dedicated to compensate for Ionospheric group delay.
A Total Electron Content (TEC) value can then be derived from a correct reception of the two signals on the two distinct carrier frequencies. For instance the frequency separation between L1 and L2 GPS signal is around 350 MHz. Ionospheric TEC is characterized by observing carrier phase delays of received radio signals transmitted from satellites located above the ionosphere, often using Global Positioning System (GPS) satellites. TEC is an important descriptive quantity for the ionosphere of the Earth and is strongly affected by solar activity. A definition of TEC is the total number of electrons integrated between two points, along a tube of one meter squared cross-section, i.e., the electron columnar number density.
TEC is significant in determining the scintillation and group and phase delays of radio waves passing through a medium. By making measurements simultaneously on signals transmitted by a satellite on two distinct frequencies, most of the detrimental effects of the ionosphere on the radio signal can be removed. By knowing the TEC, the phase dispersion can be derived. The phase dispersion may be expressed by:
                                          φ            ⁡                          (              f              )                                ⁡                      [            rad            ]                          =                              -            2                    ·          π          ·                                    40.3              ·                              10                16                            ·                              TEC                ⁡                                  [                  TECU                  ]                                                                                                      c                  0                                ⁡                                  [                                      m                    ⁢                                          /                                        ⁢                    s                                    ]                                            ·                              f                ⁡                                  [                  Hz                  ]                                                                                        Equation        ⁢                                  [        2        ]            
Where: C0 is vacuum speed of light.
There are a number of potential problems with such a dual frequency approach. For example, the additional information may not be available in some parts of the world, for instance during major conflicts. Furthermore, a presence of the second frequency channel also increases susceptibility to disturbance by a presence of high interference or jamming devices.
However, single-frequency devices, such as most vehicle navigation and handheld receivers, don't offer the opportunity of dual-frequency correction. These single-frequency devices must rely on a single-frequency, real-time, mapping correction model. Thus, a second known method to compensate for Ionospheric group delay is to use a model that represents the expected Ionospheric group delay, for example the Klobuchar (1987) model or the International Reference Ionosphere (IRI) model (Bilitza 2001). The Klobuchar method is based on an empirical approach. To use this method, the user needs to receive a Klobuchar ionospheric model broadcast by the transmitter on navigation satellite signals, which models the ionosphere delay as a function of the user position. Consequently, this second method is known to be complex to implement.
The coefficients for such a model are included in the navigation messages transmitted by all GPS satellites. Known as the Ionospheric Correction Algorithm or Klobuchar Algorithm, it removes at least 50 percent of the ionosphere's effect on radio signals passing there through. Such real-time mapping information is typically sent on a per-region basis via a geostationary satellite broadcasting on GPS frequencies. Known real-time mapping techniques are employed in the Wide Area Augmentation System (WAAS) available in North America, the European Geostationary Navigation Overlay System (EGNOS) available in Europe, and the Multi-functional Satellite Augmentation System (MSAS) available in Asia. Also, any extension of EGNOS to North Africa or the Middle East may well increase the impact of ionosphere on the new area, as the TEC value is higher when closer to the Equator.